/Engine/Source/Runtime/RenderCore/Public/VertexFactory.h
Can be overridden byFVertexFactorysubclasses which have EVertexFactoryFlags::SupportsPSOPrecaching set - this function will only be called when that flag is set
static void GetPSOPrecacheVertexFetchElements ( EVertexInputStreamType VertexInputStreamType, FVertexDeclarationElementList & Elements )